The present invention resides in a silica polymorph and a process for preparing the same.
The preparation of catalysts suitable for use in isomerization reactions and the synthesis of alkylated aromatic hydrocarbons by reacting toluene with a methylating agent is not a new concept. Catalysts and processes are available for producing various mixtures of hydrocarbons, such as, xylene isomers, polyalkylbenzenes, etc. Catalysts and methods of preparing the same, which have been used in the past to prepare xylenes, benzenes, etc. are those selected from acidic cogels, acid-promoted kieselguhr, and various crystalline zeolite aluminosilicates.
Crystalline zeolites are described as a three-dimensional network of structural units consisting of silicon-centered SiO.sub.4 and aluminum centered AlO.sub.4 of a tetrahedral configuration, the tetrahedra being interconnected by a mutual sharing of oxygen atoms, the structural arrangement of which forms cavities or cells forming crystalline channels or pore openings having a uniform diameter characteristic of each variety or type of crystalline zeolite.
The silica polymorph herein, which is a major component in aromatic alkylating or isomerization catalysts, has a topological type of crystalline structure composed of silicate tetrahedra connected in a framework to form a three-dimensional system of intersecting channels defined by 10-ring members sufficiently wide enough to absorb molecules up to 6 A in diameter. The silica polymorph is hydrophobic and organophilic, and selectively adsorbs organic molecules over water.
The silica polymorph herein is particularly suited for admixing with inorganic refractory oxides and promoters, such as, phosphorus and nitrogen compounds and the like, to prepare a catalyst useful in alkylation and isomerization reactions.
The preparation and use of silica in a catalyst which is suitable for use in aromatic alkylation and isomerization reactions is appreciated and disclosed by the prior art.
For example, U.S. Pat. No. 4,061,724 relates to a crystalline silica composition which is described as selective in adsorbing organic materials from water in either the liquid or vapor phase. The crystalline silica is described as suitable for removing organic compounds from waste water.
U.S. Pat. No. 4,002,698 teaches a process for methylating toluene to selectively produce paraxylene by contacting toluene with a methylating agent under methylation conditions in the presence of a crystalline aluminosilicate zeolite catalyst.
Another process for alkylating aromatic hydrocarbons is set forth in U.S. Pat. No. 4,025,572; particularly, the reference discloses a process for the conversion of alcohols or ethers to olefinic and aromatic hydrocarbons in the presence of a crystalline aluminosilicate zeolite. This process is described as suitable for simultaneously producing olefinic hydrocarbons and mononuclear aromatics with high selectively for para-xylene formation.
U.S. Pat. No. 3,884,835 relates to crystalline silica compositions which are synthesized from a sodium silicate or other silica source having a sodium oxide to silica oxide ration of 0.3 to 7. The chemical composition is essentially 100 percent silica with less than about 3 percent aluminum oxide and sodium oxide.
As can readily be determined from the above, there is an ongoing research effort for new and better catalysts, including crystalline silicates, for use in alkylation and isomerization reactions.